Before permanent agriculture, harmful human impacts on nature were mostly limited to the over-hunting of animals and the burning of particular habitats for game management. By 10,000 years ago, however, this all started to change as farmers began to replace existing plant communities with cultivated fields-a process that has continued ever since in response to continually expanding human population. Neither of these impacts on living communities is particularly hard to understand: if we eat too many animals, there will be less of them, or none at all, and their natural predators may also disappear; if we keep chopping down and plowing up forests and grasslands, very few of their original inhabitants will still be present as time goes on. Humanity’s third major impact on nature, however, is much less obvious, and it involves forces that are largely invisible. Its damaging events often occur hidden away inside large factories or within the tissues of living things, and understanding them requires some familiarity with science and engineering. These are the impacts that result from the creation and use of a wide range of manufactured chemicals.